Electronic devices capable of sensing magnetic field strength have become increasingly important in many fields of industry. The simplest of such devices is the magnetoresistor, which consists of a material whose resistance varies as a function of an applied magnetic field. When used in conjunction with a permanent magnet, for instance, magnetoresistors can be made into accurate position sensors. Such a device is shown in U.S. Pat. No. 4,939,456 (Donald T. Morelli et al., issued Jul. 3, 1990) entitled, "Position Sensor Including a Thin Film Indium Arsenide Magnetoresistor on a Permanent Magnet."
Magnetoresistors have a resistance that increases roughly quadratically with an applied magnetic field. It is desirable that the magnetoresistor have a high resistance. To achieve this characteristic, magnetoresistors have been made of a thin film of a suitable material on a substrate of an insulating or semi-insulating material. To achieve high electron mobility, a material having a high electron mobility, such as indium arsenide, has been used as the magnetoresistor material. Also, as described in U.S. Pat. No. 4,926,154 (Joseph P. Heremans et al., issued May 15, 1990) entitled, "Indium Arsenide Magnetoresistor," thin films, 5 micrometers or less, of indium arsenide having an accumulation layer along its surface with the areal density of the surface accumulation layer being substantially larger, at least an order of magnitude larger, than the areal density of the bulk of the film, provide excellent magnetoresistor materials. The electron accumulation layer is effective to provide a magnetic sensitivity and range of operating temperatures as if the indium arsenide thin film was apparently much thinner and had a much higher electron density and electron mobility.
Another type of magnetic sensitive device which is widely used is a Hall generator. For example, see the article of D. L. Endsley et al. entitled, "Four-Terminal Analysis of the Hall Generator," published in IRE TRANSACTIONS ON ELECTRON DEVICES, May 1961, pgs. 220-224. Hall generators give a voltage output that vary essentially linearly in a magnetic field. They are also sensitive to the polarity of the field, and at substantially low fields are more sensitive than a magnetoresistor. Therefore, Hall generators are more widely used when the polarity, as well as the sensitivity, of the magnetic field is to be measured.
The output voltage from a single Hall generator is proportional to the inverse of the carrier density per unit area of the sensor, and also to the amount of current that can be sent to the sensor. The first condition favors a semiconductor with a low carrier density. The second condition requires a good electrical conductivity. These two conditions can only be met simultaneously in a high-mobility semiconductor material. Also, sensors used in severe temperature extremes, such as those experienced near an automobile engine, require a material whose sensitivity to a magnetic field is not dependent on temperature. In principle, this can be achieved by doping the semiconductor materials to obtain a large carrier density. This solution is commonly used in magnetoresistors. However, in a Hall generator, contrarily to the case of a magnetoresistor, large carrier densities lead to a lower signal output.
Hall generators have been made from silicon. However, such Hall generators have a very low output. Although usable devices have been made by providing on-chip amplification, this requires high-gain amplifiers and this limits the usable bandwidth. The silicon technology further restricts the temperature range over which each device can work. Bulk indium arsenide and bulk indium antimonide have been used to make the most sensitive Hall generators. However, the devices are not only expensive, but are also not sensitive enough for widespread applications. Gallium arsenide with a few thousand Angstrom thick n-type conductivity doped surface layer made by ion implantation or epitaxial growth on GaAs or AlGaAs buffer layer is used to make inexpensive discrete devices that can operate at higher temperatures. However, this device lacks sensitivity because of the low mobility of gallium arsenide compared to indium arsenide and indium antimonide. Although on-chip amplification can be used and is being developed for gallium arsenide Hall generators, these devices still suffer from the lower output of the Hall generator itself. The amplifiers require high gains and this can only be achieved at the expense of band width. In automotive applications, the high frequency cut-off of these sensors can be too low.